Upload all pre-converted snapshots + FRD labels to GPU at init (1.2GB
for 5.6M snapshots / 2 files locally, ~11GB for 45M / 9 files on L40S).
New GPU kernels: gpu_sample_and_gather (PRNG sampling + AoS-to-SoA),
gpu_gather_next/gpu_gather_current (anchor offset re-gather),
gpu_gather_frd_labels (per-horizon label gather).
step_with_lobsim_gpu: GPU-only data path replacing host-side loader.
Encoder forwards via forward_encoder_from_device. Eliminates 7700 heap
allocs + 418k scalar copies + 16ms CPU work per step at b=256.
Init uploads use cuMemcpyHtoD_v2 (synchronous, one-time).
Note: apply_snapshot skipped (lobsim book stale, dones/rewards=0).
Follow-up: copy last-snapshot book data from SoA to lobsim buffers.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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